1. Technical Field
The present invention relates to a surface light source device having a function for generating a polarized light flux from a polarization-free light or a function for generating a much polarized light flux from a less polarized light (hereinafter called polarization function). The surface light source device of the invention is employed in applications requiring polarized light flux, and is particularly applied beneficially to backlight of a liquid crystal display.
2. Description of the Related Art
It has been hitherto known well to apply a surface light source device comprising a light guide plate having light scattering ability on the surface or inside, and a light source (such as fluorescent lamp) disposed at the side to backlight of a liquid crystal display or the like (for example, Japanese Laid-open Patents No. H2-221926 and No. H4-145485, and Japanese Laid-open Utility Model No. 51-89888).
In these "conventional" surface light source devices, the principle is based on the idea that the quantity of light emitted from the light exit surface is maintained by making the light propagation direction as random as possible by the light scattering action in the light guide or surface region. Therefore, the presented illuminating light is free from evident directivity, and does not have function for generating the light possessing polarization in a specific direction.
By contrast, the present inventor previously proposed a light flux parallelizing apparatus by making use of light flux parallelizing function of a wedge-shaped light scattering light guide having characteristic parameters for defining the scattering characteristic (effective scatter irradiation parameter E and correlation distance a; detail given later) determined in a specific range (Japanese Patent Application No. H5-349479, filed Dec. 28, 1993). When a modified surface light source device is constituted by combining the light flux parallelizing apparatus with a light source, the illuminating light flux has a definite directivity, but polarization polarized in a specific direction cannot be generated.
As the means for obtaining polarized light from a polarization free light emitted from an ordinary light source, polarizing plates and beam splitter type polarizer are known. These polarizing elements eliminate the polarized components in a direction orthogonal to a specific direction in order to obtain a polarized light.
As a result, about half of the light energy is lost. Therefore, if the polarizing element is combined with any one of the surface light source devices mentioned above, it is difficult to obtain polarized illuminating light flux efficiently.
On the other hand, when a polarization-free illuminating light flux is entered from behind a liquid crystal display panel of a liquid crystal display, by the polarizing plate provided at the light incident side of the liquid crystal display panel in order to enter the light polarized in a specific direction into the liquid crystal layer, about half of the light energy of the illuminating light flux generated in the surface light source device is wasted. Accordingly, the brightness of the display of the liquid crystal display is sacrificed.
If the surface light source device having a capability of generating a light flux polarized in a specific direction having a suppressed light energy loss is presented, by employing it in the backlight of the liquid crystal display, the loss of the liquid crystal display panel when passing through the polarizing plate can be lowered (The polarizing direction of the illuminating light flux from the backlight and the polarizing axis direction of the polarizing plate are set parallel.). From such viewpoint, realization of a surface light source device having a capacity of generating a light flux polarized in a specific direction with a suppressed light energy loss has been waited for.